A development in technology concerning ad hoc network which is a temporal network constituted by wireless communication terminals implemented with relay function is in progress.
In the ad hoc network, the communication terminals are on even ground with one another and act in autonomous and distributed manner, without depending on an existing particular network infrastructure, such as telephone line, portable phone network, and the Internet network. Therefore, the communication terminals (nodes) within a wireless communication area are allowed to exchange information directly via radio communication. Even when the nodes are not allowed to exchange information directly with each other, due to a location where radio wave does not reach, exchange of information is available through relaying by intermediate nodes (radio multihop communication).
In the above mentioned ad hoc network, when a closed communication network where only communication terminals belonging to a specific group are allowed to communicate with one another is constituted, it is necessary to prevent a connection from a communication terminal not belonging to the group and also to prevent leakage of communication data, in order to ensure security of the information within the group.
With regard to the security of the closed communication network, for example, Japanese Patent Laid-open Publication No. 2002-111679 (hereinafter, referred to as “Patent Document 1”) discloses a technique which ensures security, by distributing a common key for use in encryption and the like, in a group communication method which autonomously builds a closed network with unspecified number of communication terminals. Concretely, there has been suggested that PtoP (Peer to Peer) connection is established between a communication terminal as a sending source of calling message and a communication terminal being a responder, and the common key is distributed by a public key of the communication terminal on the responding side, thereby sharing the common key within the group.
In addition, EP Patent Laid-open specification No. 102430 (hereinafter, referred to as “Patent Document 2”) discloses a technique which authenticates a node that is not directly connected, when a communication terminal attempts to join the ad hoc network.